1. Technical Field
The present application relates to backup bearings and especially to backup bearings for extreme speed "touch-down" applications.
2. Background of Related Art
The flywheel, a balanced mass spinning around a constant axis that stores energy as rotational kinetic energy is one of humankind's earliest devices, serving as the basis for both the potter's wheel and the grindstone. Today, flywheel energy storage (FES) systems which store electric energy as kinetic energy and generate electric energy from the stored kinetic energy are being utilized for a number of applications. FES systems are currently being utilized in both mobile applications such as automotive and space applications, as well as stationary applications such as utility load-leveling systems, uninterrupted power supplies, and as storage capacity for solar and wind power systems. As is traditional, the term stationary refers to a system which is positioned primarily in a given geographic location as contrasted to a mobile system which is able to readily move between a variety of geographic locations.
FES systems generally include several principal components; namely a flywheel having a rotor and a hub, a motor/generator as well as magnetic bearings. Typically the system will also include a structural housing, a vacuum pump, electrical power input/output and electronic controls for the magnetic bearings.
It is known in the art to construct the flywheel rotor of high specific strength (i.e. strength/density) composite materials in order to optimize the flywheel's performance. The motor/generator is utilized to transfer electric power into the system to store it as kinetic energy when the system is acting as a motor and is also utilized to generate electric energy from the stored kinetic energy to transfer the electric energy out of the system when the system is operating as a generator. High-performance FES systems operate in a vacuum to minimize windage losses, aerodynamic heating and rotor instability. These high-performance systems therefore include a structural housing which also serves as a containment vessel to enclose any debris resulting from the failure of the rotor. Current FES systems also use magnetic bearings for supporting, or suspending the rotating flywheel within the housing.
Typically, the magnetic bearings utilized are either active or passive. In a typical active system the flywheel is suspended by magnetic forces created by the magnetic bearings. These forces, along with the loads that act on the flywheel, are controlled and balanced by proximity sensors and electronic feedback circuits working together to control the stability of the flywheel by introducing magnetic flux forces by controlling the currents in electromagnetic windings within the bearing assembly. Passive magnetic bearings, on the other hand, use powerful permanent magnets with specific geometries to support and stabilize the spinning flywheel without resorting to feedback control. Passive bearings help minimize parasitic losses while active bearings allow for more dynamic stability than passive bearings and are useful in mobile applications, such as in automobiles where compensation for road shocks and rotor balance to avoid flywheel instability is important.
Magnetic bearings work very well in situations of low energy loss and low vibration when they are properly placed, or centered, and operating. However, if the flywheel is forced off-center or if there is an interruption in the power source the bearings may not be able to restore themselves and can abruptly fail. In order to minimize damage within the system due to failure, many flywheels utilize rolling element backup bearings placed along the inner diameter of the flywheel such that if the flywheel becomes misaligned and is no longer operating on the magnetic bearings the backup bearings will take over in order to help prevent extensive damage to the entire system.
Typical rolling element backup bearings work well for spin-up under low energy situations for a short duration, but have limitations when there are repeated periods of high speed operation. "Spin-up" as used in this application refers to accelerating the stationary backup-bearing to operating speed by contact with a rotating element. Rolling element bearings typically include an inner race, rolling elements and an outer race all disposed about and stationary with respect to the stationary shaft of the FES system. The backup bearing assembly also includes an airgap located between the outer race and the rotating flywheel assembly. In a typical backup bearing system if the flywheel assembly becomes mis-aligned the flywheel assembly will impact the stationary outer race forcing the outer race to spin-up in speed with the load path going through the rolling element and the inner race. Impact of the rotating flywheel assembly with the outer race causes a skid area on the outer race, the skidding friction causing the outer race to begin rotating about the stationary shaft.
The energy transfer needed to "spin up" the rolling element in order to take over the operation of the flywheel assembly for the magnetic bearings is equal to the energy of accelerating the mass of the outer race plus the energy to accelerate the rolling elements to their operating speed. This energy is transmitted through the skidding of the outer race against the rotating flywheel assembly. The longer it takes the rolling element bearings to come up to speed the more skidding which occurs between the outer race and the flywheel assembly. In high speed applications the skid damage caused by the outer race contacting the rotating flywheel assembly results in shortened life of this contact surface in a typical backup bearing system.
A need therefore exists for a backup bearing which is capable of: 1) coming up to the operational speed of the flywheel assembly after magnetic bearing failure with little or no wear to the backup bearing device and 2) repeated periods of high speed operation after magnetic bearing failure with little wear to the backup bearing device.
The present application provides for a backup bearing system which is capable of quickly coming up to the operational speed of the flywheel assembly after magnetic bearing failure with little wear to the backup bearing and which is also able to operate for repeated "spin-up" periods at high speeds with little wear to the backup bearing system.